Air treated in these air separation installations contains a certain number of impurities. Most of them, such as hydrocarbons and nitrogen oxides, are less volatile than oxygen and accumulate in the liquid in the vaporizer. In the case of a simple column, there will be a vaporizer-head condenser in which accumulates rich liquid and in the case of a double column, there is generally a vaporizer-condenser which ensures heat exchange between the oxygen of the low pressure column and either the nitrogen of the medium pressure column, or the pressurized air, or any other cycle fluid (preferably the nitrogen at a pressure different than that of the medium pressure column).
Being less soluble (the solubility being only about several ppm), these impurities give rise, alone or in combination, to various dangers, particularly to the risk of explosion. They are therefore to be eliminated. Different means are used to prevent any accumulation, in particular removal by adsorption, in liquid or gaseous phase such as described in DE-A-19.36.049, or purging the liquid from the vaporizer.
Safety requires on the one hand preventing the accumulation of dangerous impurities, on the other hand the surveillance to make sure that these are effective and efficacious.
Until now, attention has been focussed on the hydrocarbons, particularly acetylene. Surveillance of the good operation of the elimination means is achieved by following the hydrocarbon content, in general by chromatography (for example, with detection by flame ionization), although analysis by chromatography is costly and difficult to conduct.
Nowadays, removal of carbon dioxide and water from air supplied to an installation is achieved by adsorption upstream of the cryogenic separation. A certain number of air impurities, in particular unsaturated hydrocarbons are, at least partially, removed by this purification by adsorption. Others, including some of the nitrogen oxides, continue to the cold box which contains the column or columns. The means to prevent the accumulation of these impurities in the liquid in the vaporizer can therefore be simplified, because of the presence of the adsorption system, but not removed and it is necessary to continue to monitor whether the devices for this purpose are efficacious.
No matter what the operation of the apparatus provided with an adsorption system upstream of the cryogenic separation (producing for example liquid or gaseous oxygen), the hydrocarbon content in the liquid in the vaporizer is, ordinarily, negligible and often undetectable, except for the alkanes (methane, ethane, propane), but a knowledge of their content is not of interest because the prevention devices are substantially ineffective relative to these pollutants which, moreover, are not dangerous. The non-detection of hydrocarbons does not mean for all that, that the prevention means used are efficacious because, most often, the limiting factors for their dimensioning are the nitrogen oxides. On the other hand, they are relatively difficult to measure, and the use of a chromatograph is required. Surveillance of the hydrocarbons therefore does not permit verifying, in a simple and certain manner, efficacious operation of the safety means used.